Electrolyte for an energy storage device

ABSTRACT

The invention relates to energy storage devices such as capacitors and supercapacitors and non-aqueous solvent systems suitable for use as an electrolyte solvent therein. Devices incorporating the solvent system are suitable for use in, for example, wireless devices or automotive applications at high temperatures with minimal, if any mass loss. The solvent system has at least one low boiling component (preferably a nitrile, eg acetonitrile) at least one high boiling component compatible with said low boiling component (preferably lactones, eg γ-butyrolactone and/or organic carbonates eg ethylene carbonate or propylene carbonate); and wherein the components are selected in an amount such that said non-aqueous solvent system does not boil at the boiling point of the low viscosity solvent alone but has a boiling point greater than said low viscosity solvent alone.

This application is a 371 national phase application of PCT/AU03/00334filed on 19 Sep. 2003, claiming priority to PS1195 filed 19 Mar. 2002,the contents of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The invention relates to electrolytes for use in energy storage devices.In particular, the invention relates to non-aqueous electrolytes capableof high temperature operation in capacitors and supercapacitors.

The invention has been developed primarily for supercapacitors and willbe described hereinafter with reference to that application. It will beappreciated, however, that the invention is not limited to thatparticular field of use and is also suitable for other energy storagedevices such as batteries, fuel cells, pseudocapacitors and capacitorsand hybrids of one or more of these devices.

BACKGROUND ART

Supercapacitors, alternatively known as ultracapacitors, electricaldouble layer capacitors or electrochemical capacitors, are energystorage devices that have considerably more specific capacitance thanconventional capacitors. Low resistance supercapacitors are ideallysuited for high power applications for mobile devices, particularlythose using GSM (Global System for Mobile communication) and GPRS(General Packet Radio Service) wireless technologies.

Supercapacitors can play a role in hundreds of applications. The energyand power storage markets, where supercapacitors reside, are currentlydominated by batteries and capacitors. It is well recognised thatbatteries are good at storing energy but compromise design to enablehigh power delivery of energy. It is also well recognised thatcapacitors enable fist (high power) delivery of energy, but that theamount of energy delivered is very low (low capacitance). Overlayingthese limitations of existing batteries and capacitors against marketdemand reveals the three main areas of opportunity for supercapacitors,battery replacement, devices which have higher energy density, badcomplements, devices which have high power and energy densities; andcapacitor replacement, devices which are smaller and not only have highpower density but have high frequency response.

Currently, the relatively high power density of supercapacitors makethem ideal for parallel combination with batteries that have high energydensity to ram a hybrid energy storage system. When a load requiresenergy that is not constant, complementing the battery with asupercapacitor allows the peaks to be drawn from the charged-upsupercapacitor. This reduces tie load on the battery and in many casesextends the lifecycle of a battery as well as the lifetime ofrechargeable batteries.

Modern mobile devices require power systems that arm capable of dealingwith large fluctuations in the load. For example, a mobile telephone hasa variety of modes each with a different load requirement. There is astand-by mode, which requites low power and is relatively constant.However, this mode is periodically punctuated by the need to find thenearest base station and a signal is sent and received, requiring ahigher load. In full talk mode where continuous contact to a basestation is required, the load takes the form of a periodic signal wherethe instantaneous load is quite different from the average. A number ofcommunication protocols exist, such as GSM and GPRS, but they are allcharacterized with a periodic load. The parallel supercapacitor-battyhybrid is particularly suited to this application because the power fromthe supercapacitor is used during the high loads that are usually shortin duration and the energy from the battery can recharge thesupercapacitor and supply a base load during the time of low powerdemand. As further miniaturization of digital wireless communicationdevices occur, leading to decreased battery sizes, the need forsupercapacitors will increase.

Supercapacitors also have application in the field of Hybrid ElectricVehicles (HEV). Supercapacitors can be used as an integral component ofthe drivetrains of these vehicles and are used as the primary powersource during acceleration and for storage of energy reclaimed duringregenerative braking. Such vehicles could conceivably halve a motorist'sfuel bill and slash emissions by up to 90%.

Capacitance arises when two parallel plates are connected to an externalcircuit and a voltage difference is imposed between the two plates, thesurfaces become oppositely charged. The fundamental relationship forthis separation of charges is described by the following equation

$C = \frac{ɛ\; A}{L}$where C denotes capacitance with a unit of farads (F), ε is thepermittivity with a unit of farads per metre (m), A is the area ofoverlap of the charged plates and L is the separation distance. Thepermittivity of the region between the plates is related to thedielectric constant of the material that can be used to separate thecharged surfaces.

The problem with exiting commercial capacitors using conventionalmaterials is that their performance is limited by their dimensions. Forexample, a capacitor based around a metallized coating of a polyethylenesheet that is 50 μm thick will develop only 0.425 μF for one squaremetre of capacitor. Thus, over 2.3 million square metres will berequired to develop 1 F.

The supercapacitors developed by the present applicant are disclosed indetail in the applicants copending applications, for example,PCT/AU98/00406, PCT/AU99/00278, PCT/AU99/00780, PCT/AU99/01081,PCT/AU00/00836 and PCT/AU01/00553, the contents of which areincorporated herein by reference.

These supercapacitors developed by the applicant overcome thedimensionality problem described above by using as a coating material anextremely high surface area carbon.

These supercapacitors include two opposed metal electrodes. Theseelectrodes are coated and are maintained in a predetermined spaced apartelectrically isolated configuration by an intermediate electronicallyinsulating separator. In very broad terms, the electrodes form currentcollectors for the coating material, in that the metal offerssignificantly less resistance than the coating material. The coatingstypically formed from a particulate carbon or carbons and a binder usedfor adhering the carbon to itself and to the associated currentcollector.

The coated electrodes and intermediate separator can be either stackedor wound together and disposed within a housing that contains anelectrolyte. Two current collecting terminals are then connected to andextend from respective electrodes for allowing external access to thoseelectrodes. The housing is sealed to prevent the ingress of contaminantsand the egress of the electrolyte. This allows advantage to be take ofthe electrical double layer that forms at the interface between theelectrodes and the electrolyte. That is, there are two interfaces, thosebeing formed between the respective electrodes and the electrolyte. Thistype of energy storage device is known as a supercapacitor.Alternatively, these have been known as ultracapacitors, electricaldouble layer capacitors and electrochemical capacitors.

The electrolyte contains ions that are able to freely move throughout amatrix, such as a liquid or a polymer, and respond to the chargedeveloped on the electrode surface. The double layer capacitance resultsfrom the combination of the capacitance due to the compact layer (thelayer of solvated ions densely packed at the surface of the electrode)and the capacitance due to the diffuse layer (the less densely packedions further from the electrode).

In supercapacitors, the compact layer is generally very thin, less thana nanometre, and of very high surface area. This is where thetechnological advantage for supercapacitors over conventional capacitorslies, as charge storage in the extremely thin compact layer gives riseto specific capacitances of approximately 0.1 Fm⁻². This is an increaseby several hundred thousand-fold over conventional film capacitors. Aswell, the applied potential controlled, reversible nanoscale ionadsorption/desorption processes result in a rapid charging/dischargingcapability for supercapacitors.

The electrode material may be constructed as a bed of highly porouscarbon particles with a very high surface area. For example, surfaceareas may range from 100 m² per gram up to greater than 2500 m² per gramin certain preferred embodiments. The colloidal carbon matrix is heldtogether by a binding material that not only holds the carbon together(cohesion) but it also has an important role in holding the carbon layeronto the surface of the current collecting substrate (adhesion).

The current collecting substrate is generally a metal foil. The spacebetween the carbon surfaces contains an electrolyte (frequently solventwith dissolved salt). The electrolyte is a source of ions which isrequired to form the double layer on the surface of the carbon as wellas allowing ionic conductance between opposing electrodes. A porousseparator is employed to physically isolate the carbon electrodes andprevent electrical shorting of the electrodes.

The energy storage capacity for a supercapacitor can be described by theequation

$E = {\frac{1}{2}{CV}^{2}}$where E is the energy in joules and V is the rated or operating voltageof the supercapacitor. Apart from the voltage limitation, it is the sizeof the supercapacitor that controls the amount of energy stored, and thedistinguishing feature of supercapacitors are the particularly highvalues of capacitance. Another measure of supercapacitor performance isthe ability to store and release the energy rapidly; this is the power,P, of a supercapacitor and is given by

$P = \frac{V^{2}}{4R}$where R is the internal resistance of the supercapacitor. Forcapacitors, it is more common to refer to the internal resistance as theequivalent series resistance or ESR. As can be deduced from theforegoing equations, the power performance is controlled by the ESR ofthe entire device, and this is the sum of the resistance of all thematerials, for instance, substrate, carbon, binder, separator,electrolyte and the contact resistances as well as between the externalcontacts.

One variable of interest in the field of supercapacitors that has yet tobe fully explored is the nature of the electrolyte involved. Theelectrolyte is typically one or more solvents containing one or moredissolved ionic species. In many cases, the physical and electrochemicalproperties of electrolyte are a key factor in determining the internalresistance (ESR) of the supercapacitor and the “power spectrum” of thesupercapacitor, ie the ability of the supercapacitor to provide powerover various time domains or in various frequency ranges.

The factors influencing the conductance (κ) of an electrolyte solutionare described in detail in an article by B. E. Conway taken from “TheFourth International Seminar on Double Layer Capacitors and SimilarEnergy Storage Devices”, Dec. 12-14, 1994, held at Ocean Resort Hoteland Conference Centre, Deerfield Beach, Fla. and co-ordinated by FloridaEducational Seminars, Inc., 1900 Glades Road, Suite 358, Boca Raton,Fla. 33431.

In summary, there are two principle factors which are involved indetermining the conductance—these are:

-   a) the concentration of free charge carriers, cations and anions;    and-   b) the ionic mobility or conductance contribution per dissociated    ion in the electrolyte.

There are a number of sub factors which in turn influence these twoprinciple factors. These are:

-   The solubility of the selected salt.-   The degree of dissociation into free ions and factors such as the    extent of ion-pairing of the ionic species. This in turn is    influenced by the salt concentration, temperature and the dielectric    constant of the solvent.-   The viscosity of the solvent, which is a temperature dependent    property. As temperature increases, there is a corresponding    decrease in viscosity.

Solvents for supercapacitors can thus be designed with the followingcriteria in mind:

-   Solvent for selected ionic species-   Degree of dissociation of cation/anion pairing in solution-   Dielectric constant-   Electron-pair donicity-   Permits high ion mobility-   Extent of solvation of free ions and radii of solvated ions-   Temperature coefficient of viscosity (ie low viscosity in the    intended temperature range) and ion pairing equilibria.

There is also the necessity for the solvent to be chemically stable.Aqueous based electrolytes, such as sulfuric acid and potassiumhydroxide solutions, are often used as they enable production of anelectrolyte with high conductivity. However, water is susceptible toelectrolysis to hydrogen and oxygen on charge and as such has arelatively small electrochemical window of operation outside of whichthe applied voltage will degrade the solvent. In order to maintainelectrochemical stability in applications requiring a voltage in excessof 1.5V, it is necessary to employ supercapacitor cells in series, whichleads to an increase in size in relation to non-aqueous devices.Stability is important when one considers that the supercapacitors mustcharge and discharge many hundreds of thousands of times during theoperational lifetime of the supercapacitor.

There are of course processing requirements on the solvent also, such ascost, toxicity, purity and dryness considerations.

Non aqueous solvents commonly used in related fields, eg batteries, canbe classified as: high dielectric constant aprotic (e.g. organiccarbonates), low dielectric constant with high donor number (e.g.dimethoxyethane, tetrahydrofuran or dioxolane), low dielectric constantwith high polarisability (e.g. toluene or mesitylene) or intermediatedielectric constant aprotic (e.g. dimethylformamide, butyrolactone)solvents.

However, in addition to the specific electrolyte requirements ofsupercapacitors mentioned above, there is also the practicalconsideration that supercapacitors do not operate in isolation. Rather,in use, they are in confined environments in the presence of componentswhich generate high temperatures, and like the other components, thismust be borne in mind when selecting the electrolyte solvent. Also, itneeds to be borne in mind that the supercapacitors must be capable ofoperation at start-up at temperatures much lower (even into the sub zerorange) than the high operating temperatures referred to above.

The energy storage of batteries, in contrast to the power delivery ofsupercapacitors, is not critically dependent on the contribution of theelectrolyte to the ESR of the cell, although even in batteries, low ESRis desirable. Solvents which have high boiling points invariably havehigh viscosities, and consequently, low charge mobilities at lowtemperatures. High boiling solvents, such as cyclic ethers and lactonescan therefore be used in batteries with less regard to what would be anunacceptably high ESR in supercapacitors.

FIG. 1 shows the relationship between literature boiling point andviscosity for a number of substances.

FIG. 2 shows the relationship between conductivity and reciprocalsolvent viscosity at 25° C. for 0.65M tetraethylammoniumtetrafluoroborate (TEATFB) for a variety of solvents. Source: Makoto Ue,Kazuhiko Ida and Shoichiro Mori; “Electrochemical Properties of OrganicLiquid Electrolytes Based on Quaternary Onium Salts for ElectricalDouble-Layer Capacitors.” J. Electrochem. Soc., Vol. 141, No. 11,November 1994

FIG. 3 is a plot of ESR and reciprocal conductivity, where theconductivity is varied by changing the concentration of TEATFB inacetonitrile, and shows in a general way the relationship between ESRand conductivity for a supercapacitive cell.

These three Figures also serve to illustrate the other relationshipsthat exist between the properties, such as boiling point and ESR,viscosity and ESR and boiling point and conductivity.

Admixing a low boiling fluid and a high boiling fluid may appear to bean attractive option, with the low boiling, low viscosity compoundproviding acceptable charge mobility at the low end of the temperaturerange, and the high boiling component reducing in viscosity andproviding charge mobility at higher temperatures. In practice, however,this approach is generally not viable because while acceptable resultsmay be achieved at ambient temperatures, at higher temperatures the lowboiling component will fractionate out. Fractionation can present achallenge to the mechanical integrity of the supercapacitor packaging.

It is an object of the present invention to provide a non-aqueoussolvent suitable for use in the energy storage device which overcomesone or more of the abovementioned disadvantages, or at least provides acommercially viable alternative.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a non-aqueoussolvent system suitable for use as an electrolyte solvent in an energystorage device, said non aqueous solvent system including:

-   at least one low boiling component,-   at least one high boiling component compatible with said low boiling    component; and-   wherein the components are selected in an amount such that said    non-aqueous solvent system does not boil at the boiling point of the    low viscosity solvent alone but has a boiling point greater than    said low viscosity solvent alone.

Alternatively, the invention may be described as providing a non-aqueoussolvent system suitable for use in an energy storage device including aplurality of compatible component solvents each with a correspondingcomponent solvent boiling point, and wherein the non-aqueous solventsystem has at least one boiling point not corresponding to a componentsolvent boiling point.

Preferably the energy storage device is a supercapacitor. Morepreferably, the energy storage device is a carbon based supercapacitor,that is, a supercapacitor that has carbon as a component of theelectrodes.

The energy storage devices of the present invention may be in the formof cells or devices, and may include a number of cells in series orparallel.

Preferably, the non-aqueous solvent system is a combination of a lowviscosity solvent and one or more compatible high viscosity solvents.

Preferably the low viscosity/low boiling component is a nitrile, mostpreferably acetonitrile (“AN”).

The high viscosity/high boiling component is preferably one or more of alactone, such as γ-butyrolactone (“GBL”), or an organic carbonate suchas ethylene carbonate (“EC”), propylene carbonate (“PC”) or mixtures orderivatives thereof.

Preferably, the species are complexed or associated and provide asynergistic change in boiling point. Preferably, the species are in amole ratio selected to provide an electrolyte solvent with a boilingpoint different from the boiling point of the low viscosity solvent.

In one preferred embodiment, the sum of the moles of the low boilingcomponents is less than the sum of the moles of the high boilingcomponents. In an alternative preferred embodiment, the sum of the molesof the low boiling components is equal to the sum of the moles of thehigh boiling components. In another alternative preferred embodiment,the sum of the moles of the low boiling components is greater than thesum of the moles of the high boiling components.

In a preferred embodiment, the invention provides a non-aqueous solventsystem suitable for use as an electrolyte solvent in an energy storagedevice, said non aqueous solvent system including:

-   a nitrile,-   at least one of a lactone or a carbonate compatible with said    nitrile; and wherein the components are selected in an amount such    that said non-aqueous solvent system does not boil at the boiling    point of the nitrile but has a boiling point greater than the    boiling point of the nitrile.

In one particularly preferred embodiment, the invention provides anon-aqueous solvent system including acetonitrile, γ-butyrolactone, andethylene carbonate. Even more preferably, the invention provides anon-aqueous solvent system including acetonitrile, γ-butyrolactone, andethylene carbonate in a mole ratio of 3:2:1 to 3:1.72:1.

In another particularly preferred embodiment, the invention provides anon-aqueous solvent system including acetonitrile, γ-butyrolactone, andpropylene carbonate. Even more preferably, the invention provides anon-aqueous solvent system including acetonitrile, γ-butyrolactone, andpropylene carbonate in a mole ratio of 3:2:1 to 3:1.72:1.

In yet another particularly preferred embodiment, the invention providesa non-aqueous solvent system including acetonitrile, propylene carbonateand ethylene carbonate. Even more preferably, the invention provides anon-aqueous solvent system including acetonitrile, propylene carbonateand ethylene carbonate in a ratio of 2:1:1.

Other preferred embodiments include 2AN:GBL:PC and 2AN:GBL:EC

Without wishing to be limited to the particular solvents which may beused, the high boiling high viscosity solvents and/or low boiling lowviscosity solvents may be selected independently from the followinglist. It will be understood that high boiling and low boiling, andlikewise high viscosity and low viscosity, are relative terms andrepresent properties of the component solvents relative to one another.

Suitable solvents include: ethylene carbonate, propylene carbonate,butylene carbonate, γ-butyrolactone, γ-valerolactone, acetonitrile,glutaronitrile, adiponitrile, methoxyacetonitrile,3-methoxypropionitrile, N,N-dimethylformamide, N,N-dimethylacetamide,N-methypyrrolidinone, N-methyloxazolidinone,N-N′-dimethylimisazolidinone, nitromethane, nitroethane, sulfolane,dimethyl sulfoxide, trimethyl phosphate, 1,2-dimethoxyethane,tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane,4-methyl-1,3-dioxolane, methyl formate, methyl acetate, methylpropionate, dimethyl carbonate, ethyl methyl carbonate, diethylcarbonate, methyl propyl carbonate, 1-methyl-2-pyrrolidone,1,2-dichloroethane, sulphuryl chloride, thionyl chloride, acetylchloride, tetrachloroethylene carbonate, benzoyl chloride,dichloroethylene carbonate, nitrobenzene, acetic anhydride, phosphorusoxychloride, benzonitrile, selenium oxychloride,propanediol-1,2-carbonate, benzylcyanide(nitrile), ethylene sulphite,iso-butyronitrile, propionitrile, phenylphosphonic difluoride,n-butyronitrile, acetone, ethyl acetate, phenylphosphonic dichloride,diethyl ether, diphenyl phosphonic chloride, trimethyl phosphate,tributyl phosphate, pyridine, hexamethyl phosphoramide and the like.

The non-aqueous solvent systems of the present invention have a boilingpoint of at least 85° C., more preferably at least 90° C. and even morepreferably at least 100° C.

Preferably, the non-aqueous solvent systems of the present inventionfurther include an ionic species at least partially soluble therein,such as a salt, which may be in one preferred embodimenttetraethylammonium tetrafluoroborate. The ionic species may be presentin an amount up to saturation, or in greater or lesser quantities suchas 1 molar and in an amount sufficient to allow an energy storage deviceto function over the desired temperature range.

Preferably, the non-aqueous solvent systems of the present inventioninclude an ion source. The ion source may be present in an amount up tosaturation at −30° C. or in an amount up to saturation at anytemperature, having regard to the operational requirements of thedevice. Tetraethylammonium tetrafluoroborate is particularly preferred.In one highly preferred embodiment, a 1 molar (at ˜23° C.) solution oftetraethylammonium tetrafluoroborate in the solvents of the presentinvention have a conductivity of at least 40 mS/cm at 85° C., morepreferably at least 50 mS/cm at 85° C., even more preferably at least 55mS/cm at 85° C. and most preferably at least 60 mS/cm at 85° C. It ispreferable that the non aqueous systems of the present invention aresuitable four use as high temperature solvents and/or low temperaturesolvents.

According to a second aspect, the invention provides a method ofincreasing the boiling point of a non-aqueous low boiling low viscositysolvent suitable for use in an energy storage device, said methodincluding the step of combining said non-aqueous low boiling lowviscosity solvent with at least one compatible high boiling highviscosity solvent.

According to a third aspect, the invention provides a method ofdecreasing the viscosity of a high boiling high viscosity solventsuitable for use in an energy storage device, said method including thestep of combining said high boiling high viscosity solvent with at leastone compatible second liquid.

According to a fourth aspect, the invention provides a method ofincreasing the useful operational temperature range of a solventsuitable for use in an energy storage device, said method including thestep of combining a low boiling low viscosity solvent with at least onecompatible high boiling high viscosity solvent.

According to a fifth aspect, the invention provides a high temperaturesolvent suitable for use in an energy storage device, said hightemperature solvent including acetonitrile, γ-butyrolactone and ethylenecarbonate. In an alternative embodiment, the high temperature solventincludes acetonitrile, γ-butyrolactone and propylene carbonate. Afurther alternative embodiment of the high temperature solvent includesacetonitrile, propylene carbonate and ethylene carbonate.

According to a sixth aspect, the invention provides a low temperaturesolvent suitable for use in an energy storage device, said lowtemperature solvent including acetonitrile, γ-butyrolactone and ethylenecarbonate. In an alternative embodiment, the low temperature solventincludes acetonitrile, γ-butyrolactone and propylene carbonate. Afurther alternative embodiment of the low temperature solvent includesacetonitrile, propylene carbonate and ethylene carbonate.

According to a seventh aspect, the invention provides an energy storagedevice including the non-aqueous solvent system of the presentinvention. In one preferred embodiment the device includes acetonitrile,γ-butyrolactone and ethylene carbonate.

In an alternative embodiment, the energy storage device may include asolvent including acetonitrile, γ-butyrolactone and propylene carbonate.A further alternative embodiment of the energy storage device mayinclude a solvent including acetonitrile, propylene carbonate andethylene carbonate.

Preferably the energy storage device is a capacitor or supercapacitor.

According to an eighth aspect, the invention provides a method ofpredetermining the ESR of an energy storage device at a predeterminedtemperature, said method including the step of providing to the energystorage device a solvent system including at least one low boilingcomponent, at least one high boiling component compatible with said lowboiling component; and wherein the components are selected in an amountsuch that said non-aqueous solvent system does not boil at the boilingpoint of the low viscosity solvent alone but has a boiling point greaterthan said low viscosity solvent alone.

According to a ninth aspect, the invention provides a method ofpredetermining the conductivity of an energy storage device at apredetermined temperature, said method including the step of providingto the energy storage device a solvent system including at least one lowboiling component, at least one high boiling component compatible withsaid low boiling component; and wherein the components are selected inan amount such that said non-aqueous solvent system does not boil at theboiling point of the low viscosity solvent alone but has a boiling pointgreater than said low viscosity solvent alone.

The preferred solvent systems include, but are not limited to:acetonitrile, γ-butyrolactone and ethylene carbonate; acetonitrile,γ-butyrolactone and propylene carbonate or acetonitrile, ethylenecarbonate and propylene carbonate.

According to a tenth aspect, the invention provides a supercapacitorhaving an ESR of no more than 1013 mΩ cm² at 23° C., preferably no morethan 862 mΩ cm² at 23° C. and even more preferably no more than 449 mΩcm² at 23° C. and an ESR of no more than 7840 mΩ cm at −30° C.,preferably no more than 3685 mΩ cm² at −30° C. and even more preferablyno more than 986 mΩ cm² at −30° C.

Where ESR is described in terms of resistance multiplied by unit area,it will be understood by those skilled in the art that this refers tothe geometric area of the current collector. Also, in those cases wherethe devices have differently sized current collectors, it will beunderstood that the resistance values relate to the area of the smallestcurrent collector.

According to an eleventh aspect, the invention provides a supercapacitorhaving an ESR of no more than 784 mΩ cm² at −85° C., preferably no morethan 670 mΩ cm² at 85° C. and even more preferably no more than 508 mΩcm² at 85° C. and an ESR of no more than 7840 mΩ cm² at −30° C.,preferably no more than 3685 mΩ cm² at −30° C. and even more preferablyno more than 778 mΩ cm² at −30° C.

According to a twelfth aspect, the invention provides a supercapacitorhaving an ESR of no more than 784 mΩ cm² at 85° C., preferably no morethan 670 mΩ cm² at 85° C. and even more preferably no more than 508 mΩcm² and an ESR of no more than 946 mΩ cm² at 23° C. and preferably nomore than 862 mΩ cm² at 23° C. and even more preferably no more than 449mΩ cm² at 23° C.

According to a thirteenth aspect, the invention provides asupercapacitor having an ESR of no more than 784 mΩ cm² at 85° C. andpreferably no more than 670 mΩ cm² at 85° C. and even more preferably nomore than 508 mΩ cm² at 85° C. and an ESR of no more than 946 mΩ cm² at23° C., preferably no more than 862.4 mΩ cm² at 23° C., and even morepreferably an ESR of no more than 544 mΩ cm² at 23° C. and an ESR of nomore than 7840 mΩ cm² at −30° C. and preferably no more than 3685 mΩ cm²at −30° C. and even more preferably no more than 778 mΩ cm² at −30° C.

According to a fourteenth aspect, the invention provides asupercapacitor having an ESR of no more than 771 mΩ cm² at 80° C.,preferably no more than 424 mΩ cm² at 80° C.

According to a fifteenth aspect, the invention provides a supercapacitorhaving an ESR of no more than 741 mΩ cm² at 90° C., preferably no morethan 412 mΩ cm² at 90° C.

According to a sixteenth aspect, the invention provides a supercapacitorhaving an ESR of no more than 717 mΩ cm² at 100° C., preferably no morethan 401 mΩ cm² at 100° C.

According to a seventeenth aspect, the invention provides asupercapacitor having an ESR of no more than 675 mΩ cm² at 120° C.,preferably no more than 382 mΩ cm² at 120° C.

According to an eighteenth aspect, the invention provides asupercapacitor having an ESR of no more than 657 mΩ cm² at 130° C.,preferably no more than 373 mΩ cm² at 130° C.

According to a nineteenth aspect, the invention provides asupercapacitor having an ESR of no more than 641 mΩ cm² at 140° C.,preferably no more than 366 mΩ cm² at 140° C.

The supercapacitors of the present invention may have any combination ofone or more of the ESR/temperature relationships mentioned above.

In one highly preferred aspect of the invention, the supercapacitorshave an ESR of no more than (((1044.3/(0.3948*(T)+25.852))+6.5178)*28)[Series X with 50 μm Separator] and more preferably no more than(((777.58/(0.3948*(T)+25.852))+6.741)*28) [Series Z with 50 μmSeparator] and even more preferably no more than(((649.32/(0.3948*(T)+25.852))+8.7202)*28) [Series Z with 20 μmSeparator] where all units are in mΩ cm² at temperature T(° C.).

In an alternative aspect, where the device is a multilayer electrodestack device, as may be preferred in production cells, the ESR ispreferably no more than (((1051.2/(0.3948*(T)+25.852))+13.282)*24.4) mΩcm².

These values are applicable for single cell devices. Where two or morecells are connected in series, a much higher value in mΩ cm² will beobtained.

Preferably, the supercapacitors are high temperature supercapacitors.

According to a twentieth aspect, the invention provides a supercapacitorhaving a non aqueous solvent system and an ESR at −30° C. of no morethan. 7.4, more preferably no more than 4.5, even more preferably nomore than 3.4 and most preferably no more than 2.0 times the ESR at −30°C. of a supercapacitor of identical construction but which containsacetonitrile as sole solvent.

The non aqueous solvent systems are preferably binary or ternary.

According to a twenty first aspect, the invention provides asupercapacitor having a non aqueous solvent system and an ESR at −20° C.of no more than 2.7, more preferably no more than 2.2, even morepreferably no more than 2.1 times the ESR at −20° C. of a supercapacitorof identical construction but which contains acetonitrile as solesolvent.

According to a twenty second aspect, the invention provides asupercapacitor having a non aqueous solvent system and an ESR at 23° C.of no more than 1.8, more preferably no more than 1.5, even morepreferably no more than 1.2 times the ESR at 23° C. of a supercapacitorof identical construction but which contains acetonitrile as solesolvent.

According to a twenty third aspect, the invention provides asupercapacitor having a non aqueous solvent system and an ESR at 50° C.of no more than 2.0, more preferably no more than 1.5, even morepreferably no more than 1.4 times the ESR at 50° C. of a supercapacitorof identical construction but which contains acetonitrile as solesolvent.

According to a twenty fourth aspect the invention provides asupercapacitor having a non aqueous solvent system and an ESR at −30° C.of no more than 13.7, more preferably no more than 8.3, even morepreferably no more than 6.4 and most preferably no more than 3.5 timesthe ESR at 23° C. of a supercapacitor of identical construction butwhich contains acetonitrile as sole solvent.

According to a twenty fifth aspect, the invention provides asupercapacitor having a non aqueous:solvent system and an ESR at −20° C.of no more than 4.4, more preferably no more than 3.6, even morepreferably no more than 3.4 times the ESR at 23° C. of a supercapacitorof identical construction but which contains acetonitrile as solesolvent.

According to a twenty sixth aspect, the invention provides asupercapacitor having a non aqueous solvent system and an ESR at 50° C.of no more than 1.6, more preferably no more than 1.3 times the ESR at23° C. of a supercapacitor of identical construction but which containsacetonitrile as sole solvent.

According to a twenty seventh aspect, the invention provides asupercapacitor having a non aqueous solvent system and an ESR at 85° C.of no more than 1.4, more preferably no more than 1.2, and mostpreferably no more than 1.1 times of the ESR at 23° C. of asupercapacitor of identical construction but which contains acetonitrileas sole solvent.

The supercapacitors of the present invention may have any or all of therelative performance properties of the tenth to twenty seventh aspects.

According to a twenty eighth aspect, the invention provides a method ofselecting a solvent system for use in an electrical storage deviceincluding the steps of:

-   selecting a plurality of potential solvents;-   preparing a primary, binary, ternary or higher order mixture of said    potential solvents, optionally adding an ion source;-   determining a property of said primary, binary, ternary or higher    order mixture; preparing a phase diagram of said mixtures; and-   identifying a solvent mixture adapted to provide a predetermined    value of said property.

Preferably, the binary, ternary or higher order mixture includes atleast one high boiling high viscosity solvent and at least one lowboiling low viscosity solvent.

Preferably, the binary, ternary or higher order mixture is a combinationof a low viscosity solvent and one or more compatible high viscositysolvents.

Preferably the low viscosity solvent is a nitrile, most preferablyacetonitrile.

Preferably the high viscosity solvent is one or more of a lactone, suchas y butyrolactone, or an organic carbonate such as ethylene carbonate,propylene carbonate or derivatives thereof.

Preferably, the given parameter is one or more of boiling point,conductivity, viscosity or ESR at a predetermined temperature.

According to a twenty ninth aspect the invention provides asupercapacitor, preferably of a multilayer soft packaging laminatedesign, which has a mass loss of no more than 3% of the room temperaturemass on sustained heating at 100° C., preferably a mass loss of no morethan 2% of the room,temperature mass and even more preferably a massloss of no more than 1% of the room temperature mass. Sustained heatingis preferably a period in excess of 2 hours continuous use.

According to a thirtieth aspect the invention provides a supercapacitor,preferably of a multilayer soft packaging laminate design, which has amass loss of no more than 2% of the room temperature mass on sustainedheating at 95° C., preferably a mass loss of no more than 1% of the roomtemperature mass and even more preferably a mass loss of no more than0.5% of the room temperature mass. Sustained heating is preferably aperiod in excess of 3 hours continuous use.

According to a thirty first aspect the invention provides asupercapacitor, preferably of soft packaging laminate design, which hasa mass loss of no more than 0.5% of the room temperature mass onsustained heating at 90° C. and even more preferably zero mass loss onsustained heating at 90° C. Sustained heating is preferably a period inexcess of 4 hours continuous use.

According to a thirty second aspect, the invention provides asupercapacitor having an extrapolated ESR at infinite electrolyteconductivity (ESR_(∝)) of no more than 325 mΩ cm², more preferably nomore than 189 mΩ cm² and most preferably no more than 147 mΩ cm².

In another aspect the invention relates to a device incorporating anenergy storage device of the present invention. Such devices include,but are not limited to devices such as digital wireless devices, forexample, mobile telephones. Devices of the present invention alsoinclude computers, and related combination devices which may benetworked conventionally or in a wireless manner. Other devices are inthe form of an electrical vehicle or hybrid electrical vehicle. It willbe appreciated that the devices of the present invention are especiallysuited to those applications where high temperature use is expected, butwhere design considerations would render bulky “can” typesupercapacitors unsuitable.

The energy storage devices of the present invention maybe used, forexample, with a GPRS communications module for a cellular telephone, aGSM module, a Mobitex module, 3G module, a PCMCIA card, a Compact Flashcard, a communications card or device for a notebook computer, a laptopcomputer or a Tablet computer, a wireless LAN device such as a desktopor other computer or any other wireless device.

Most preferably, the device of the present invention is a supercapacitorused as part of a power source in a PCMCIA card, especially a modem orfax modem card.

Preferably, when the energy storage device of the present invention areused with communications modules or cards, they are in the form of asupercapacitor having a plurality of supercapacitive cells. The cellsare preferably connected in series and even more preferably, the cellsare contained within the same package, although the cells may becontained within separate packages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of 1/boiling point against 1/viscosity for a rangeof solvents.

FIG. 2 shows a graph of conductivity against 1/viscosity for a range ofsolvents.

FIG. 3 shows a graph of ESR against 1/conductivity (as a function ofTEATFB concentration at 23° C.) for Series Y standard test cells withacetonitrile.

FIG. 4 shows a graph of ESR against temperature for mono solvent systemsfor Series Y and Series X standard test cells.

FIG. 5 shows the relationship between conductivity and ESR for a SeriesX standard test cell with acetonitrile.

FIG. 6 shows a graph of ESR vs 1/conductivity (obtained by varying thetemperature) for a Series X standard test cell with acetonitrile.

FIG. 7 collates FIGS. 6 and 3 to allow comparison of the concentrationand temperature effects.

FIG. 8 shows lines of best fit for conductivity against temperature forthree electrolyte systems, namely 1M TEATFB in 3AN:1.72GBL:EC,3AN:2GBL:EC and AN.

FIG. 9 shows ESR against temperature for standard test cells with 1MTEATFB in 3AN:1.72GBL:EC (Series X and Z), 3AN:2GBL:EC (Series Z) and AN(Series X). The separator is nominally 50 μm thick.

FIG. 10 shows ESR against temperature for Series Z standard test cellshaving 1M TEATFB in 3AN:2GBL:EC and AN electrolytes. The separator isnominally 20 μm thick.

FIG. 11 shows ESR against 1/conductivity for standard test cells with 1MTEATFB in 3AN:1.72GBL:EC (Series X and Z), 3AN:2GBL:EC (Series Z) and AN(Series X). The separator is nominally 50 μm thick.

FIG. 12 shows ESR against 1/conductivity for Series Z standard testcells having 1M TEATFB in 3AN:2GBL:EC and AN electrolytes. The separatoris nominally 20 μm thick.

FIG. 13 shows ESR verses temperature for a multiple layered electrodestack in a single cell and also for two of these cells connected inseries to form a two cell device. The electrolyte used was 1M TEATFB in3AN:1.72GBL:EC. The separator was nominally 50 μm thick.

FIG. 14 shows ESR verses 1/conductivity for a multiple layered electrodestack in a single cell and also for two of these cells connected inseries to form a two cell device. The electrolyte used was 1M TEATFB in3AN:1.72GBL:EC. The separator was nominally 50 μm thick.

FIGS. 15 to 26 show phase diagrams for electrolyte mixtures. The phasediagrams show the mole ratios of the solvent mixture. The property ofthat particular solvent mixture is represented in boldface text, initalics or underlined.

FIGS. 27 and 28 show thermogravimetric analysis of a multilayered softpackaging laminate supercapacitor cell with prior art electrolyte(acetonitrile). The cell in this figure takes the form of a multiplelayered electrode stack.

FIGS. 29 and 30 show thermogravimetric analysis of a multilayered softpackaging laminate supercapacitor cell with an electrolyte of thepresent invention (3AN:1.72GBL:EC). The cell in this figure takes theform of a multiple layered electrode stack

FIGS. 31 and 32 show thermogravimetric analysis of a multilayered softpackaging laminate supercapacitor device with an electrolyte of thepresent invention.

FIG. 33 shows a contour plot of boiling points for AN:PC:GBL ratios.

DESCRIPTION

The present invention is described with reference to the supercapacitorsdeveloped by the present applicant and disclosed in detail in theapplicants copending applications, for example, PCT/AU98/00406,PCT/AU99/00278, PCT/AU99/00780, PCT/AU99/01081, PCT/AU00/00836 andPCT/AU01/00553. It will be understood by those skilled in the art thatthe present application uses those supercapacitors and that in thepresent instance, the solvent is the variable of interest. However, itwill also be appreciated by those skilled in the art that theelectrolyte solutions of the present application will be equallyapplicable for use in other energy storage devices of different design.

Acetonitrile (AN) is widely used as the sole solvent component ofelectrolyte systems because it has a high dielectric constant (38 at 20°C.) and a low viscosity (0.369 cP at 20° C.). A 1M solution oftetraethylammonium tetrafluoroborate has a room temperature conductivityof 55 mS/cm, which is around 2-5 times better than can be attained usingmost other single component organic solvents.

Acetonitrile also has a low freezing point and relatively low viscosity,making it suitable for low temperature applications. However,acetonitrile boils at 82° C. which means that at or above thistemperature it is necessary to contain the vapour, and additionalchallenges need to be met in respect of ensuring the mechanicalintegrity of any packaging of devices which use AN at or above thistemperature.

A thermogravimetric analysis of a supercapacitor cell containingacetonitrile made without any special consideration to containing highpressure shows a sudden and irreversible weight loss at 83° C. In someapplications, an upper temperature limit of 80-85° C. is unsatisfactory,and higher temperatures (up to 95° C. and above) are required forprolonged periods.

As mentioned, it is important that the electrolyte has as high aconductivity and as low a contribution to device ESR as possible. Highconductivity can be achieved primarily by using a low viscosity (or, inpractical terms, low boiling) solvent, although in order for highconductivity, it is also necessary for the solvent to have a gooddielectric constant to enable it to dissolve ionic species.

As mentioned above, simply employing a single compound with a higherboiling solvent is not desirable for various reasons. While a number ofhigher boiling solvents are available with good dielectric constants,they are invariably significantly more viscous than lower boilingsolvents. Further, high viscosity solutions do not exhibit appropriateconductivities until much higher temperatures (where the viscosity isreduced). Thus, while these high temperature solvents are capable ofgood conductance at high temperature, they have unsatisfactorily highESRs at ambient or subzero temperatures. To illustrate the problem, atroom temperature the conductivity of acetonitrile is around 55 mS/cmwhile that of γ-butyrolactone (GBL) is only around 18 mS/cm.Conductivity increases with temperature but the conductivity ofγ-butyrolactone does not approach the room temperature conductivity ofacetonitrile until the temperature reaches 130° C. To those skilled inthe art, admixing the two would not appear to produce a solution to theproblem as acetonitrile, which boils around 80° C. would be expected tofractionate out of the mixture long before a suitable conductivity ofγ-butyrolactone was achieved. Surprisingly, in the present case, suchfractionation did not occur.

In particular, the present applicant has found that solvent blends, suchas a blend of a nitrile, a lactone and a carbonate, and in particularacetonitrile, γ-butyrolactone and propylene carbonate (PC) or a blend ofacetonitrile, γ-butyrolactone and ethylene carbonate (EC) or a blend ofacetonitrile, propylene carbonate (PC) and ethylene carbonate (EC)produce a ternary solvent that has good conductivity, (and consequentlya suitably low ESR) over a good temperature operating range, with highstability at elevated operational temperature, such as 85° C.

Without wishing to be bound by theory, it is believed that thisstability at elevated temperatures is due to association between thespecies, i.e. rather than being a mere admixture which undergoesfractionation of the lower boiling components as temperature increases,an association between the species means that the acetonitrile does notfractionate out of the mixture. The fact that no fractionation occurredleads to the hypothesis that acetonitrile forms complexes with the othersolvent molecules in the solution which results in the elevation of theacetonitrile boiling temperature, i.e. the formation of a new complexwith a higher boiling point.

A 1M Tetraethylammonium tetrafluoroborate solution in a mole ratio of 3acetonitrile: 1.72-2 γ-butyrolactone:1 ethylene carbonate mixtureperformed unexpectedly well in the tests as is illustrated in theexamples. This ternary mixture had a boiling temperature of 109° C. withno fractionation of acetonitrile around its boiling point as would havebeen expected.

Thermo gravimetric analysis of supercapacitor test cells and devicescontaining 3 acetonitrile: 1.72 γ-butyrolactone:1 ethylene carbonateshowed that the weight of the material remained constant up to at least103° C. before sample loss occurred. It was highly significant that nosample loss commenced immediately above 82° C., the boiling point ofacetonitrile. Such an observation bears out the hypothesis of someintermolecular interactions taking place.

Further, qualitative analysis of samples containing the ternary system,but with a significant mole excess of acetonitrile showed somefractionation, indicating that beyond a certain point, there was nofurther opportunity for complexation of the acetonitrile.

Further investigations as to the mechanism of the temperature elevationwithout fractionation were conducted and in particular whether or not itinvolved some solvation of the ionic species in solution. Depending onthe solvent and particular ionic species, the addition of a dissolvedsalt can generally increase boiling temperature by around 1-3° C. permole of ionic species. For example, the boiling point of anotherpreferred ternary solvent (2AN:0.86 GBL:EC) of the present invention wasaround 107° C. without the salt. Adding a salt to a concentration of 1 Mgave a boiling point of around 108-113° C., an increase of up to 6° C.This corresponds to a rise of up to 3° C. per mole of ionic specieswhich is within the expected,limits.

By contrast, the difference between the boiling point of the mixture andthe boiling point of pure acetonitrile is around 25° C. There is strongevidence that the mixture is more than merely an admixture, but rather asolution in which there is an interaction between the species.

EXAMPLES General Procedure

In order to identify those solvent systems stable over an extendedlifetime at elevated temperatures (≧85° C.), the following generalprocedure was adopted.

Dried, recrystallised TEATFB was used throughout. Solvents used in thisexperiment were obtained from Merck Germany with the highest qualityavailable i.e. Selectipur® and were run through a chromatography columnpacked with about 10 cm of γ alumina. The moisture content in the finalproduct was estimated by Karl Fischer titration as follows: GBL=10 ppm,PC=5 ppm, AN=2 ppm. EC was a solid and was not further purified. Oncethe salt was added, the mixture was shaken well until all salts weredissolved.

Where ratios of solvents were used, these refer to mole ratios.

Solutions of TEATFB were all 1 molar unless otherwise indicated. Whereexperiments are conducted on solvent only (eg, AN, or 2AN:0.86 GBL:EC)this is indicated in the text.

The solvent mixtures were prepared with final volumes between 30 to 40ml which were sufficient for boiling point and conductivity tests.

The conductivity of these electrolytes were measured inside a drynitrogen atmosphere in a glove box using a handheld ULTRAMETER (Model6P) from Nyron L Company in accordance with the recommended procedure inthe operating manual.

For boiling point determination, the sample vial was filled with about20 ml of test electrolyte plus some boiling chips and heated rapidly(˜10° C./min) until the temperature reached ˜75° C., then reduced to arate rise of about 2° C./min or less, with continued monitoring of thesolution.

EC, being a solid at room temperature, was kept in a 50° C. environmentto ensure it remained liquid at all times. Where EC was used inconjunction with other solvents in a binary or ternary mixture, the saltwas added subsequent to the combining of the solvents.

Unless otherwise stated, a standard test cell of area 28 cm² was used togenerate results. For the standard test cells, two carbon-coatedelectrodes were cut to a size of 28 cm² excluding terminals. Theelectrodes are cut such that they are 8 cm×3.5 cm. The terminals wereapproximately 4 cm long and were 2.5 cm from the corner along thelongest edge. One electrode was folded in half such that the carbon wasfacing inwards. The second electrode was folded in half such that thecarbon was facing outwards. This second electrode was encompassed in amembrane separator and the membrane-encased electrode was slid into thefirst electrode. Unless stated otherwise a 50 μm polyolefin membrane wasused. Those skilled in the art will appreciate that both the materialsand the thickness of the membranes can be varied considerably withouteffecting the overall functionality of the device. The carbon layerswere facing each other with a separator in between. The device wasassembled so that the terminals were both pointing in the samedirection.

A multilayer soft packaging laminate was wrapped around the electrodesallowing the terminals to protrude to the outside of the packet. Thepacket was heat sealed leaving one end open. The cell was dried usingheat and vacuum. The packet was filled with enough electrolyte to coverthe electrodes and sealed. The sealed packet was pierced and taken to atight vacuum. The packet was sealed again close to the electrode stackto complete the standard test cell.

Examples of electrode arrangements may be found in our copendingapplications PCT/AU01/01613 and PCT/AU01/01590, the contents of whichare incorporated herein by reference.

The cell was then cycled between a low voltage and the voltage at whichthe cell was to be used. Electrical testing was then performed. ESRmeasurements were taken, at voltage as per the industry standard, whichin the present case is 1.8V, at 1 kHz. Capacitance was measured using adischarge current of 0.2 A.

Where the following data is dependent upon the construction of thesupercapacitor, such data is given as being either “Series X”, which hasa nominal 4.5 μm carbon layer; “Series Z” which has a nominal 7.3 μmcarbon layer thickness; “Series Y”, which has a nominal 10 μm carbonlayer thickness; and “Series W” which has a 13.5 μm coating thickness.The density of the series is as follows: Series X-0.22 mg/cm³; SeriesY-0.33 mg/cm³; Series W-1.12 mg/cm³ and Series Z 0.35 mg/cm³. Becausethe series data relate to variations in the construction of thesupercapacitor Series X data should only be compared with other Series Xdata and so on. Control data obtained for acetonitrile in all seriesenables the relative results to be standardised and compared. The cellswhich take the form of a multiple layered electrode stacks invariablyused a coating thickness of nominally 6 μm and a density ofapproximately 0.35 mg/cm³

Experimental errors in observed values have not been quoted here,although those skilled in the art will be familiar with the precisionand accuracy with which such values are normally determined.

Descriptions of the construction of multilayered electrode stack devicesare disclosed in our copending application PCT/AU01/01613, the contentsof which are incorporated herein by reference. In the present case, theelectrode area was 24.4 cm².

The standard test cell, for a nominally 6 μm thick coating and nominally50 μm thick separator membrane, has a volume in the order of 1.23×10⁻⁶m³ and a weight of 1.76 g including the multilayer packaging laminate.

The standard test cell, for a nominally 6 μm thick coating and nominally50 μm thick separator membrane, has a volume in the order of 3.03×10⁻⁷m³ and a weight of 0.43 g neglecting the multilayer packaging laminate.

The cell comprised of a multiple layered electrode stack, for anominally 6 μm thick coating and nominally 50 μm thick separatormembrane, has a volume in the order of 8.62×10⁻⁷ m³ and a weight of 0.97g including the multilayer packaging laminate.

The cell comprised of a multiple layered electrode stack, for anominally 6 μm thick coating and nominally 50 μm thick separatormembrane, has a volume in the order of 3.19×10⁻⁷ m³ and a weight of 0.39g neglecting the multilayer packaging laminate.

The two cell device comprised of two multiple layered electrode stacksconnected in series, for a nominally 6 μm thick coating and nominally 50μm thick separator membrane, has a volume in the order of 1.72×10⁻⁶ m³and a weight of 1.94 g including the multilayer packaging laminate.

The two cell device comprised of two multiple layered electrode stacksconnected in series, for a nominally 6 μm thick coating and nominally 50μm thick separator membrane, has a volume in the order of 6.37×10⁻⁶ m³and a weight of 0.78 g neglecting the multilayer packaging laminate.

It will be obvious to those skilled in the art that altering thephysical properties, including the density of the coating, the thicknessof the coating, the density of the separator, the thickness of theseparator and or the density of the multilayer soft packaging laminateor the thickness of the multilayer soft packaging laminate or thethickness or density of the current collector will alter the volume andthickness of the cells similarly.

1. Mono Solvent Systems

As mentioned earlier, acetonitrile is an extremely useful electrolytesolvent. It has a very low viscosity and a very high dielectricconstant. Both these attributes combine to make an acetonitrileelectrolyte which has a very high conductivity. The downside of usingacetonitrile as the electrolyte in a supercapacitor is the fact that itboils at around 80° C. which means that there are additional containmentproblems to address if the supercapacitor is to be used at hightemperatures.

In order to identify an alternate solvent with a comparableconductivity, the parameters for likely mono solvent systems wereestablished before focussing on binary and ternary solvent systems.Three different solvents were mixed with tetraethylammoniumtetrafluoroborate up to saturation or 1M, which ever is the lesser.These mixtures were then purified in the usual method and tested in avariety of methods including electrical testing, in standard test cells,as well as conductivity measurements over a range of temperatures.

Results

Three main electrolyte solvents were tested: γ-butyrolactone (GBL),propylene carbonate (PC) and ethylene carbonate (EC). Acetonitrile wasalso used as a control.

The relevant physical properties of the solvents in question are asfollows:

Density Melting (g/cm³) Viscosity Point/Boiling Dielectric at (cP)Solvent Point (° C.) Constant 20° C. at 25° C. Acetonitrile  −46/82 38(at 20° C.) 0.78 0.369 (AN) γ-butyrolactone  −44/204-6 39 (at 25° C.)1.13  1.17 (GBL) Propylene  −48/242 65 (at 25° C.) 1.21  2.8 (20° C.)Carbonate (PC) Ethylene 35-8/247-9 95 (at 25° C.) 1.41  1.92 (40° C.)Carbonate (EC)Acetonitrile (AN):

Acetonitrile Temperature (deg C.) Conductivity (mS/cm) 1M TEATFB −2032.8 0 48.1 25 59.6 50 70.2 75 79.7As mentioned in the introduction, conductivity, viscosity, temperatureand ESR are related. FIG. 4 shows ESR versus Temperature for PC, GBL andAN. In order to illustrate the principle further, FIGS. 5 and 6 showplots of ESR against conductivity and ESR against 1/conductivityrespectively for AN.

The following data was obtained in a standard test cell:

Series Y

ESR Capacitance (0.2 A) Electrolyte (mΩ)23° C. (F) 23° C. AN 1M TEATFB25.1 0.72Series X

ESR Capacitance (0.2 A) Electrolyte (mΩ)23° C. (F) 23° C. AN 1M TEATFB20.5 0.52γ-Butyrolactone (GBL): The saturation point for this liquid, withrespect to tetraethylammonium tetrafluoroborate, is around 0.92M at roomtemperature (23° C.). The conductivity measurements over a range oftemperatures is shown in the following table:

γ-Butyrolactone Temperature (Deg C.) Conductivity (mS/cm) 0.92M −5.6 9.00.1 10.5 23.6 16.9 85.2 33.2 131.0 51.0

It can be seen from this table that the conductivity of the solutiondoes not rival the room temperature conductivity of 1M acetonitrile (55mS/cm) until over 130° C. This is most likely due to the increasedviscosity of the GBL as compared to AN. The test cells at roomtemperature (below) also show a proportionally higher ESR than thecontrol.

Series Y

ESR Capacitance (0.2 A) (F) Electrolyte (mΩ) 23° C. 23° C. AN   1MTEATFB 25.1 0.72 GBL 0.92M TEATFB 62.1 0.70Propylene Carbonate (PC): Propylene carbonate can solvate slightly morethan one molar of tetraethylammonium tetrafluoroborate. The saturationlimit is around 1.2M at room temperature. The conductivity data wasfound to be as follows:

PC 1M Temperature (Deg C.) Conductivity (mS/cm) −22.5 1.3 0 8.0 25 13.885 30.2 180 55.1

Like GBL, propylene carbonate does not have a conductivity anywhere nearthe room temperature conductivity of AN until it reaches 180° C. Theaverages for the ESR of the test cells were found to be:

Series Y

ESR Capacitance Electrolyte (mΩ) 23° C. (0.2 A) 23° C. AN 1M TEATFB 25.10.72 PC 1M TEATFB 65.4

Interestingly the dielectric constant of propylene carbonate is higherthan acetonitrile (almost double in fact) which should allow it todissociate more salt. While such a characteristic is desirable the maindrawback with using propylene carbonate, which corresponds to the higherESR, is its exorbitantly high viscosity: PC is over 7 times more viscousthan AN. The main benefit with PC is its 242° C. boiling point.

Ethylene Carbonate (EC) is slightly different from the other solventsystems used in that it is a solid at ambient temperatures.Consequently, it was not possible to obtain data for EC alone attemperatures below about 35-40° C.

The ESR of Series X and Series Y cells is given in the following tableand a plot of ESR against temperature is shown in FIG. 4.

ESR at specified temp (mΩ) −20° C. 23° C. 50° C. 85° C.   1M TEATFB inAN  33.5 20.5 18.5   1M TEATFB in PC 293.9 65.4 40.0 0.92M TEATFB in GBL62.1 44.1 EC Solid Solid2. Binary Solvent System

Following a thorough analysis of the boiling points and conductivitiesof various combinations of acetonitrile (AN), ethylene carbonate (EC),γ-butyrolactone (GBL), and propylene carbonate (PC), binary mixtures ofeach were prepared to investigate their suitability for high temperatureapplication.

The main binary systems investigated were those with a combination of alow boiling, non viscous liquid, and a higher boiling more viscousliquid. In particular, these were: AN:GBL, AN:0.86 GBL, AN:PC, and AN:EC

The electrolytes were made up as 1M (tetraethylammoniumtetrafluoroborate) TEATFB solutions and underwent electrical performanceand stability testing across a range of −20° C. to 95° C.

Control data for AN is given and those skilled in the art will readilyappreciate that this value can be used to standardize the data betweenSeries X, Series Y and Series Z and allow a direct comparison of thequantitative differences between the two data sets, should this bedesired.

Conductivity Tested temp Boiling Point Solution (mS/cm) (° C.) (° C.)0.86GBL:AN 31.3 29.0 108-110 GBL:AN 30.6 23.8 106 0.86GBL:2AN 38.1 26.4 97 GBL:2AN 36.9 23.0  97 1.72GBL:AN 25.9 26.9 125-126 2GBL:AN 24.8 23.0121 PC:AN 27.0 26.2 112 PC:2AN 26.7 26.4 112 2PC:AN 21.3 27.3 131-132PC:2.5AN 36.0 28.9  96 PC:3AN 37.7 29.4  92 EC:AN 28.5 26.2 110-113EC:2AN 43.3 26.2  93 2EC:AN 28.5 27.1 113 EC:1.5AN 32.0 30.5 104

The ESR and Capacitance of supercapacitors incorporating the solventsystems of the present invention were investigated at 23° C. The controldata and results are summarised below and are plotted on the phasediagrams and in FIG. 5.

Series X (Control)

Capacitance (0.2 A) (F) Electrolyte ESR (mΩ) 23° C. 23° C. AN 1M TEATFB20.5 0.52Series Y (Control)

Capacitance (0.2 A) (F) Electrolyte ESR (mΩ) 23° C. 23° C. AN 1M TEATFB25.1 0.72Series Z (Control)

Capacitance (0.2 A) (F) Electrolyte ESR (mΩ) 23° C. 23° C. AN 1M TEATFB19.4 0.74Series X

Electrolyte ESR (mΩ) Capacitance (F) AN:0.86GBL 1M TEATFB 26.8 0.40Series Z

Electrolyte ESR (mΩ) Capacitance (F) AN:GBL 1M TEATFB: 31.9 0.75Series Z

Electrolyte ESR (mΩ) Capacitance (F) AN:0.86GBL 1M TEATFB 27.5 0.75Series X

Electrolyte ESR (mΩ) Capacitance (F) AN:EC 1M TEATFB 35.9 0.38Series X

Electrolyte ESR (mΩ) Capacitance (F) AN:PC 1M TEATFB 38.3 0.32Series Y

Electrolyte ESR (mΩ) Capacitance (F) AN:PC 1M TEATFB 41.6 0.48

The results for the mixtures were plotted on phase diagrams, as shown inFIGS. 15 to 26.

The ESR of various binary mixtures was measured at a range oftemperatures, and the results are shown in the following table.

1M TEATFB

ESR at specified temp (mΩ) Data Series Electrolytes: −20° C. 23° C. 85°C. Series X AN:0.86GBL 70.7 26.8 23.4 Series Z AN:0.86GBL 27.5 Series ZAN:GBL 65.1 31.7 21.7 Series X AN:EC 230.3 35.9 28.3 Series X AN:PC 38.3Series Y AN:PC 41.6 27.3

The conductivity of AN:0.86GBL and AN:GBL solutions with 1M TEATFB wasdetermined for a range of temperatures. The results are shown in thefollowing table.

AN:0.86GBL Temperature (° C.) Conductivity (mS/cm) 1M TEATFB −30 13.4−20 17.0 0 24.3 23 31.8 50 42.4 85 55.2 AN:GBL Temperature (° C.)Conductivity (mS/cm) 1M TEATFB −30 10.9 −20 13.3 0 19.8 23 30.6 50 39.485 52.5Ternary Solvent Systems

A number of ternary solvent mixtures were prepared. The selection of themost likely solvent mixtures and ratios was in part based upon theresults obtained from plotting the binary mixtures around the outerperiphery of the triangular phase diagrams shown in the Figures.

The conductivity and boiling point of the electrolytes prepared areshown in the following table:

Solvent system Conductivity Tested temp (1M TEATFB) (mS/cm) (° C.)Boiling Point (° C.) PC:AN:0.86GBL 23.2 30.8 132 PC:AN:GBL 23.0 23.0122-124 PC:2AN:0.86GBL 29.0 28.0 101-105 PC:2AN:GBL 28.3 24.2 106-1083AN:0.86GBL:PC 32.4 31.0 104 3AN:GBL:PC 32.6 23.0  98 3AN:1.72GBL:PC28.7 29.9 109 3AN:2GBL:PC 28.1 23.9 109 6AN:0.86GBL:2PC 35.1 28.9  986AN:GBL:2PC 34.3 23.0  96 EC:2AN:0.86GBL 30.5 27.7 108-113 EC:2AN:GBL31.4 23.8 108 0.86GBL:EC:AN 25.6 29.9 130 GBL:EC:AN 26.4 23.0 118-1203AN:1.72GBL:EC 30.5 32.1 109 3AN:2GBL:EC 30.9 23.7 107-1103AN:0.86GBL:2EC 30.0 32.3 108-110 3AN:GBL:2EC 31.6 23.2 107 EC:AN:PC22.4 27.8 106-107 PC:EC:2AN 28.3 29.3 108-110 3AN:EC:PC 31.7 28.7101-104 4.5AN:2EC:PC 32.0 28.7 *104  6AN:2PC:EC 34.4 29.0 *100 

Those entries in the table above marked with an asterisk exhibited someapparent fractionation before reaching the stated boiling point. Withoutwishing to be bound by theory, it is believed this was as a result ofexcess acetonitrile in those mixtures over and above that required toprovide the true high boiling ternary mixture.

Boiling point elevation was also seen when AN was blended with differentmole ratios of PC, EC and GBL. Without wishing to be bound by theory,these observations lead to the hypothesis that the AN may form complexeswith the other solvent molecules in the solution which resulted in theelevation of acetonitrile boiling temperature. It was also noticed thatthe boiling temperature increased as the conductivity (at any giventemperature) of the solution decreased.

From the results above, some promising systems were chosen for ESR andcapacitance testing because they appear to have the temperature rangeand conductivities to meet ESR requirements across the temperature rangefrom −30° C. to 95° C.

The following results were obtained with standard test cells.

Series X:

Solvent 23° C. System ESR (mΩ) Capacitance (F) 3AN:1.72GBL:PC 1M TEATFB31.7 0.44 3AN:0.86GBL:2EC 1M TEATFB 30.3 0.46 2AN:PC:EC 1M TEATFB 34.50.42 2AN:0.86GBL:PC 1M TEATFB 34.0 0.41 2AN:0.86GBL:EC 1M TEATFB 31.50.43 3AN:1.72GBL:EC 1M TEATFB 30.5 0.42Series Z:

Solvent 23° C. System ESR (mΩ) Capacitance (F) 3AN:2GBL:PC 1M TEATFB33.8 0.71 3AN:GBL:2EC 1M TEATFB 31.6 0.70 2AN:GBL:PC 1M TEATFB 34.9 0.702AN:GBL:EC 1M TEATFB 31.6 0.72 3AN:2GBL:EC 1M TEATFB 26.7 0.72

A number of trials were also conducted using Series Y standard testcells. Series X and Series Y results are compared in the followingtable. All averages are based on 2-5 cells.

ESR and Capacitance 23° C. Capacitance Capacitance ESR (mΩ) (F) ESR (mΩ)(F) Electrolyte: Series Y Series Y Series X Series X 2AN:0.86GBL:EC 41.40.8 31.5 0.48 Average AN:PC:0.86GBL 48.5 0.78 Average:Series W and Series Z results for standard test cells are compared inthe following table. Averages are based on results from 5 cells.

ESR and Capacitance 23° C. ESR (mΩ) Capacitance (F) ESR (mΩ) Capacitance(F) Electrolyte: Series W Series W Series Z Series Z 2AN:GBL:EC 30.01.13 31.6 0.72 AN:PC:GBL 39.1 1.32 32.8 0.70

The ESR of the ternary mixtures were measured at varying temperatures.The results are the average of 3-5 standard test cells in Series X andSeries Z and are shown in the tables below and in FIG. 8.

Series X:

ESR at specified temp (mΩ) Solvent System −30° C. −20° C. 23° C. 50° C.85° C. 2AN:0.86GBL:EC 135.0 74.5 31.5 27.1 26.3 2AN:0.86GBL:PC 187.977.9 34.0 26.4 25.3 2AN:PC:EC 149.2 90.9 34.5 29.8 28.5 3AN:1.72GBL:EC130.6 70.6 30.5 26.0 25.5 3AN:0.86GBL:2EC 280.1 73.8 30.3 26.4 24.63AN:1.72GBL:PC 170.4 73.1 31.7 26.9 24.6Series Z:

ESR at specified temp (mΩ) Solvent System −30° C. −20° C. 23° C. 50° C.85° C. 2AN:GBL:EC 85.64 74.05 35.05 32.83 23.3 2AN:GBL:PC 89.79 59.0234.79 33.59 22.3 3AN:2GBL:EC 64.3 53.8 26.7 22.5 20.9 3AN:GBL:2EC 83.8070.10 31.57 29.72 23.1 3AN:2GBL:PC 96.46 73.15 32.71 31.66 22.28The ESR of the ternary mixtures at varying temperatures for Series X andSeries Z were adjusted for geometric area and a value of ESR multipliedby square cm of current collector (ESRx28 cm²)at different temperatureswas obtained and is shown below in the table.Series X:

Boiling Point Solvent ESR × Area at specified temp (mΩ cm²) (° C.)System −30° C. −20° C. 23° C. 50° C. 85° C. 108-113 2AN: 3763 2117 862784 706 0.86GBL:EC 101-105 2AN: 5253 2195 941 706 706 0.86GBL:PC 108-1102AN:PC:EC 4155 2509 941 862 784 109 3AN: 3684 1960 862 706 7061.72GBL:EC 108-110 3AN: 7840 2038 862 706 706 0.86GBL: 2EC 109 3AN: 47822038 862 784 706 1.72GBL:PCSeries Z:

Boiling Point Solvent ESR × Area at specified temp (mΩ cm²) (° C.)System −30° C. −20° C. 23° C. 50° C. 85° C. 108 2AN:GBL: 2489.2 2113.41012.6 907.2 637.1 EC 107-110 3AN:2GBL: 1800.4 1506.4 747.6 630 585.2 EC107 3AN:GBL: 2230.8 1876.7 901.7 821.1 606.8 2EC 109 3AN:2GBL: 2812.52097.9 945.4 880.0 619.3 PC

The ESR of the ternary mixture was compared with the ESR of acetonitrileat a range of temperatures. In this way, the relative performance of themixtures can be evaluated in a manner independent of deviceconstruction. The table below shows the ratio of the ESR of a ternaryelectrolyte device to the ESR of a corresponding acetonitrileelectrolyte device, where both devices are at the temperature specifiedin the table. The ratio for embodiments using a binary electrolyte isalso given. For reference, the absolute value of the ESR of the ANcontrol device was 38.0 mΩ at −30° C., 33.5 mΩ at −20° C., 20.5 mΩ at23° C. and 18.5 mΩ at 50° C. for series X. For the series Z device, theabsolute value of the ESR of the AN control device was 35.2 mΩ at −30°C., 31.2 mΩ at −20° C., 19.4 mΩ at 23° C. and 16.4 mΩ at 50° C.

Series X:

Boiling Point Solvent ESR of ternary/ESR of AN (° C.) System −30° C.−20° C. 23° C. 50° C. 108-113 2AN:0.86GBL:EC 3.5 2.2 1.5 1.5 101-1052AN:0.86GBL:PC 4.9 2.3 1.7 1.4 108-110 2AN:PC:EC 3.9 2.7 1.7 1.6 1093AN:1.72GBL:EC 3.4 2.1 1.5 1.4 108-110 3AN:0.86GBL:2EC 7.4 2.2 1.5 1.4109 3AN:1.72GBL:PC 4.5 2.2 1.5 1.5 108-110 AN:0.86GBL 2.1 1.2Series Z:

Boiling Point Solvent ESR of ternary/ESR of AN (° C.) System −30° C.−20° C. 23° C. 50° C. 108 2AN:GBL:EC 2.4 2.4 1.8 2.0 106-108 2AN:GBL:PC2.8 2.5 1.8 2.0 107-110 3AN:2GBL:EC 2.0 2.2 1.6 1.7 107 3AN:GBL:2EC 2.22.1 1.6 1.7 109 3AN:2GBL:PC 2.8 2.4 1.7 1.8 106 AN:GBL 2.4 2.1 1.6 1.8

It is not possible to compare the ESR of ternary electrolytes against ANat temperatures much in excess of the boiling point of AN. However, inorder to be able to compare the relative performances of all the ternaryelectrolytes (and the AN:0.86GBL, and AN:GBL binary mixtures) atelevated temperatures, they have been compared in the following tablesagainst the ESR of AN at room temperature for series X (where theabsolute value of the ESR of the AN control device at room temperaturewas 20.5 mΩ) and series Z (where the absolute value of the ESR of the ANcontrol device at room temperature was 19.4 mΩ).

Series X:

Boiling Point Solvent ESR of ternary/ESR of AN @ room temp (° C.) System−30° C. −20° C. 23° C. 50° C. 85° C. 108-113 2AN: 6.6 3.6 1.5 1.3 1.30.86GBL:EC 101-105 2AN: 9.2 3.8 1.7 1.3 1.2 0.86GBL:PC 108-110 2AN:PC:EC7.3 4.4 1.7 1.5 1.4 109 3AN: 6.4 3.4 1.5 1.3 1.2 1.72GBL:EC 108-110 3AN:13.7 3.6 1.5 1.3 1.2 0.86GBL: 2EC 109 3AN: 8.3 3.6 1.5 1.3 1.21.72GBL:PC 108-110 AN: 3.4 1.2 1.1 0.86GBLSeries Z:

Boiling Point Solvent ESR of ternary/ESR of AN @ room temp (° C.) System−30° C. −20° C. 23° C. 50° C. 85° C. 106-108 2AN:GBL: 5.1 4.0 1.8 1.61.2 PC 106-108 2AN:GBL: 5.1 4.0 1.8 1.6 1.2 PC 107-110 3AN:2GBL: 3.5 3.51.6 1.5 1.1 EC 107 3AN:GBL: 4.0 3.4 1.6 1.5 1.1 2EC 109 3AN:2GBL: 5.03.7 1.7 1.6 1.1 PC 106 AN:GBL 4.3 3.2 1.6 1.5 1.2

Trials of the 3AN:1.72GBL:EC and 3AN:2GBL:EC ternary mix electrolytesdemonstrated desirable ESR's across all temperature ranges. Mostimportantly, these cells appear to be quite stable at temperatures above85° C.

The relationship between conductivity and temperature for AN and3AN:1.72GBL:EC and 3AN:2GBL:EC is shown in FIG. 8. The continuingrelationship between conductivity and ESR can be seen to continuesmoothly to temperatures in excess of 100° C.

FIG. 8 demonstrates the suitability of the solvent for use attemperatures in excess of those attainable for acetonitrile, as well asillustrating the low ESR values which are attained using the ternarymixtures of the present invention. It is notable that the solventmixtures of the present invention provide ESR's at high temperature thatare similar to the ESR's which can be obtained from AN at roomtemperature.

FIG. 9 Shows ESR against temperature while FIG. 11 shows 1/conductivityagainst ESR. The deviation in FIG. 9 at elevated temperatures isbelieved in that case to be due to a decrease in porosity of theseparator at above 90° C. A decrease in porosity results in an increasein the resistivity of the separator.

In combination, FIGS. 8 to 11 illustrate that the solvent mixture of thepresent invention actually behaves in the same manner as a singlesolvent. Fractionating systems, with non-interacting components, wouldnot provide the seamless electrochemical behaviour over such a widetemperature range and especially over a temperature range which includesthe boiling point of AN, a major component of the mixture.

When measured in a 28 cm² test cell, ESR and temperature for the hightemperature electrolyte 3AN:1.72GBL:EC were found to be related by thefollowing equation:ESR=((1044.3/(0.3948*(T)+25.852))+6.5178) [50 μm Separator]ESR=((777.58/(0.3948*(T)+25.852))+6.741) [50 μm Separator, series z)ESR=((649.32/(0.3948*(T)+25.852))+8.7202) [20 μm Separator]

Where the, temperature T is in degrees Celsius and the ESR is in mΩ.

The relationship between ESR and temperature for AN (calculated) wasalso quantified and found to be:ESR=((1002.4/(0.4461*(T)+45.223))+5.2336) [50 μm Separator]ESR=((673.91/(0.4461*(T)+45.223))+6.7856) [20 μm Separator]

The equations were derived by plotting conductivity versus temperatureand the inverse of conductivity versus ESR for each of the two solvents.A straight line fit was placed though each data set. The lines of bestfit can be seen in FIGS. 8 and 11. The R² values for the curve fit wasfrom about 0.96 to in excess of 0.99. The linear equations were thenequated using the assumption that the conductivities are equal at anygiven temperature. The formula was then rearranged so as to be given interms of ESR vs. temperature. The ESR can then be multiplied by the areaof the smallest opposed electrode (or the area of mutual overlap betweenelectrodes, if there is some offset) to give a value of ESR cm². Themore general equation is written thus:ESR=(((1044.3/(0.3948*(T)+25.852))+6.5178)*28) [50 μm Separator]ESR=(((777.58/(0.3948*(T)+25.852))+6.741)*28) [50 μm Separator, SeriesZ]ESR=(((649.32/(0.3948*(T)+25.852))+8.7202)*28) [20 μm Separator]ESR=(((1002.4/(0.4461*(T)+45.223))+5.2336)*28) [50 μm Separator]ESR=(((673.91/(0.4461*(T)+45.223))+6.7856)*28) [20 μm Separator]

where the units for the above equations are: mΩ cm²

The plot in FIG. 11 can also be used to extrapolate an ESR value at apoint where 1/conductivity equals zero, ie ESR at infinite conductivity.Using the lines of best fit from FIG. 11, for the AN series X line anESR at infinite conductivity, ESR_(∞)=5.2336 mΩ, or when adjusted forarea, 147 mΩ cm². Similarly, for AN series Y (FIG. 7), ESR_(∞)=6.823 mΩ,or when adjusted for area, 191 mΩ cm². The ESR_(∞) from the3AN:1.72GBL:EC line was 6.741 mΩ, or when adjusted for area, 189 mΩ cm².ESR_(∞) is a useful parameter for comparing devices.

Similar equations can be constructed for other electrolyte systems, andfor differing cell constructions. For example, FIGS. 7, 9, 11, 13 and 14illustrate differences in observed values which are effected bysupercapacitor construction.

For example, in a standard test cell as disclosed above, the variationin separator thickness attributed to moving between a 20 μm separatorand a 50 μm separator.

50 μm Separator Electrolyte Equation 1M AN ESR = (((1002.4/(0.4461 * T +45.223)) + Series X 5.2336) * 28)mΩ cm² 1M 3AN:2GBL:EC ESR =(((646.94/(0.4009 * T + 22.646)) + Series Z 8.8613) * 28)mΩ cm² 1M3AN:1.72GBL:EC (1) ESR = (((777.58/(0.3948 * (R4) + 25.852)) + (1)Series Z 6.741) * 28) mΩ cm² (2) Series X (2) ESR = (((1044.3/(0.3948 *(N4) + 25.852)) + 6.5178) * 28) mΩ cm²

20 μm Separator Electrolyte Equation 1M AN ESR = (((673.91/0.4461 * T +45.223)) + Series Z 6.7856) * 28) mΩ cm² 1M 3AN:2GBL:EC ESR =(((501.19/(0.4009 * T + 22.646)) + Series Z 9.9452) * 28) mΩ cm² 1M3AN:1.72GBL:EC ESR = (((649.32/(0.3948 * (B4) + 25.852)) + Series Z8.7202) * 28) mΩ cm²

1M 3AN:1.72GBL:EC Multiple Layered Electrode Stack Cells Single cell:ESR = (((1051.2/(0.3948 * (T) + 25.852)) + 13.282) * 24.4) mΩ cm² Twocells connected in ESR = (((2045/(0.3948 * (T) + 25.852)) + series:13.009) * 48.8) mΩ cm²

The boiling point of the electrolyte with 3AN:1.72GBL:EC or 3AN:2GBL:ECternary solvent system was found to be significantly higher than that ofAN alone. This electrolyte system also had good conductivity at the highand low ends of the temperature range of interest.

Based on the boiling point and performance in the test cell, anextensive analysis of the results revealed that the 1M TEATFB in3AN:1.72GBL:EC—3AN:2GBL:EC was the preferred choice and this electrolytesolution was prepared to use in further testing.

By way of example, the following shows the method of calculation of theactual values used for the production of electrolyte as follows:

3  AN:  1.72GBL:  EC ≡ 3 × 41.05  g  AN(1  Molar  TEATFB):  1.72 × 86.09  g  GBL(0.92  Molar  TEATFB):  88  g  EC(0  Molar  TEATFB) = 123.15  g  AN:  148.08  g  GBL:  88  g  EC  Total  volume ∼ 352.6  ml

Extra salt (TEATFB) added ˜15.876 g to make total salt concentration inmixture to 1 Molar TEATFB.

The moisture in this electrolyte was removed by putting approximately100 g of γ alumina into this electrolyte and stirring well for oneminute. The alumina was allowed to settle before being filtered out.

The final moisture found in the electrolyte was measured through KarlFischer titration to be ˜16 ppm.

Stability results

The stability of multilayer soft packaging laminate devices of thepresent invention was tested by thermogravimetric analysis in aDMT-Thermo Balance under a flowing air atmosphere. For this test thecells take the form of a multiple layered electrode stacks.

Temperature was ramped at 0.1° C. per minute from ambient temperature.

The TGA shows the acetonitrile-only capacitors venting electrolytesolvent occurs between 83° C. and 86° C., see FIGS. 27 and 28 which showthe TGA results, including temperature and weight loss profiles. Bycontrast, the supercapacitor cells, FIGS. 29 and 30, and devices, FIGS.31 and 32, of the present invention having 3AN:1.72GBL:EC solventsystems showed no loss until over about 100° C.

In combination with the low ESR over a wide temperature range, the TGAstability demonstrates the suitability of the solvents systems of thepresent invention to provide stable devices with desirable power windowsover a wide temperature range.

SUMMARY

As stated earlier, the objective of the present applicants was todetermine an electrolyte which would be stable at elevated temperatureswhilst retaining a usable ESR at lower temperatures (at least −20° C.).Initially this was thought to be unrealisable when using acetonitrile,as the boiling point of acetonitrile is only 82° C. Trials wereperformed and an unusual and unprecedented trend was seen—devices withmixtures of acetonitrile managed to survive a period of time attemperatures greater than or equal to 85° C. Apparently, a boiling pointelevation phenomenon was being achieved.

There are two non-limiting theories on how this boiling point elevationcould be realised. The first is that the elevation is a manifestation ofthe effect of salt in a solution. This is a well-established theory. Theboiling point elevation due to salt is generally of the range of ˜1-3°C. per mole of ionic species in solution. The second explanation is thatthere is complexation or association between the solvents which leads toan increase in boiling point.

An experiment to distinguish between these explanations was conductedusing one mixture with and without salt. Select results have beenreproduced below.

Solution Boiling Point ° C. 2AN:0.86GBL:EC + 1M TEATFB 108-1132AN:0.86GBL:EC (Solvent only) 107 AN (solvent only)  82

It can be seen from the results above that the effect of adding salt tothe 2AN:0.86GBL:EC mixture is to increase boiling point by about 1-6° C.That is up to 3° C. per mole of ionic species. This is within thetheoretical limits of what has previously been seen on the addition ofsalt.

By contrast the difference between the mixture of 2AN:0.86GBL:EC and thepure acetonitrile is 25° C. If the mixture is not an actual solutionthen one would expect to see some fractionation at 82° C. The fact thatthis is not seen implies that there is indeed a solvation effect on theacetonitrile.

Hence it implies that, whilst the addition of salt does raise theboiling point somewhat, the main boiling point elevation is due to themixture effect.

While the invention has been illustrated with TEATFB, any other solublesalts may be used, eg Lithium, Sodium, Potassium salts and the like. Thefollowing table shows the boiling point elevations observed in a3AN:2GBL:EC mixture incorporating alternative electrolyte salts.

Boiling Point of Alternative Salts in 3AN:2GBL:EC

Boiling Salt in 3AN:2GBL:EC point (° C.) Solvent only 104-106   1MTetrabutylammonium Perchlorate 107   1M TetrabutylammoniumTetrafluoroborate 105-107   1M Tetrabutylammonium Hexafluorophosphate107   1M Triethylmethylammonium Tetrafluoroborate 108 0.5M LithiumTetrafluoroborate 106

The ternary phase diagrams summarise the results of room temperatureconductivity, room temperature ESR, ESR at low temperatures and boilingpoint elevation for solvent mixtures of acetonitrile, propylenecarbonate and ethylene carbonate; acetonitrile, propylene carbonate andγ-butyrolactone; and acetonitrile, ethylene carbonate andγ-butyrolactone.

FIG. 33 shows how the trends in a value of a particular property, egboiling point, may be evaluated. By creating a “contour plot” in whichexperimental date of equal value (ie equal boiling point) are joined, itbecomes possible to predict other intermediate solvent compositionswhich may have that boiling point, or determine which other compositionsmay have a suitable boiling temperature. While this has been exemplifiedfor boiling point elevation in AN:PC:GBL, those skilled in the art willappreciate that it can be applied equally to other solvent systems, andto other properties which depend upon the composition of theelectrolyte, such as ESR and conductivity.

The ternary phase diagrams clearly show that the attempt to find a hightemperature electrolyte is a trade off between high boiling point/highviscosity (and resultant low conductivity) on the one hand and highconductivity with a low boiling point on the other. Unfortunately theultra high temperature electrolytes have low conductivity because theyhave a high viscosity, as discussed in the introduction and shown inFIGS. 1 to 3.

The unexpected synergy of the solvent components, apparently as a resultof complexation, allows for the selection of electrolyte solvents whichhave a better performance profile over a wide range of components thanwould be predicted from looking at the component solvents alone.

1. A non-aqueous solvent system suitable for use as an electrolytesolvent in an energy storage device, said non-aqueous solvent systemincluding, by molar ratios 1 part carbonate, 2 to 3 parts acetonitrileand 0.43 to 2 parts γ-butyrolactone, and wherein the carbonate isselected from the group consisting of ethylene carbonate and propylenecarbonate.
 2. A non-aqueous solvent system according to claim 1including, by molar ratios: 3 parts acetonitrile;1.72 to 2 partsγ-butyrolactone; and 1 part ethylene carbonate.
 3. A non-aqueous solventsystem according to claim 1 including by molar ratios: 3 partsacetonitrile; 1.72 to 2 parts γ-butyrolactone; and 1 part propylenecarbonate.